Israel's invasion of Gaza has strengthened the hand of extremists and only a credible independent investigation into alleged wrongdoing can quiet growing Palestinian anger, a United Nations aid official said on Friday.

John Ging, head of the UN Relief and Works Agency (UNRWA) in Gaza, called for new U.S. Middle East envoy George Mitchell to talk to ordinary people in Gaza as part of a "new track" in diplomacy.

U.S. President Barack Obama named Mitchell, a former U.S. Senator who helped settle the conflict in Northern Ireland, on Thursday to try to jump-start Arab-Israeli peace talks."My first request to the U.S. administration is talk to the ordinary people in Gaza. Come to Gaza and talk to the ordinary people - the mothers, fathers, leaders of civil society, the people who are not involved in politics," Ging, speaking from Gaza, told reporters in Geneva.

"They are still quite shell-shocked but there is more and more anger growing."

It is urgent to establish accountability for death and the destruction of Palestinian infrastructure through a credible mechanism which would "channel this emotion to confidence in the rule of law", Ging said.

"The extremists here - there are more now at the end of this conflict than there were at the start, that's the product of such conflict - are very confident in their rhetoric that there should be no expectation that justice will be delivered through the rule of law. Now we must prove that wrong," he said.

The investigation had to examine "legitimate allegations" on both sides, as Israeli civilians had also suffered, he said.

"But it is a challenge we must succeed in achieving. Because if we don't, then we have truly conceded to the agenda of the extremists here in Gaza," he added.

U.N. Secretary-General Ban Ki-moon has said he expected Israel to provide urgently a full explanation of attacks on UN facilities in Gaza, including schools used as shelters, and said those responsible must be held accountable.

Israeli attacks killed 1,300 people and made thousands homeless in the 22-day assault which Israel said was to stop Hamas firing rockets at southern Israel. Hamas and Israel declared ceasefires on Sunday and Israel has withdrawn.

Ging, who is Irish, welcomed Mitchell's appointment.

"An individual of his experience and ability coming now to this conflict gives me cause for more than hope, it actually gives me cause for optimism that we will move on to a new track where we will see real progress," he said.

"What we hope will happen is that the U.S. administration will listen to the people. There has to be a rebalancing of the focus," he said.